Conformal thermal interface material for electronic components

ABSTRACT

A thermally-conductive interface for conductively cooling a heat-generating electronic component having an associated thermal dissipation member such as a heat sink. The interface is formed as a self-supporting layer of a thermally-conductive material which is form-stable at normal room temperature in a first phase and substantially conformable in a second phase to the interface surfaces of the electronic component and thermal dissipation member. The material has a transition temperature from the first phase to the second phase which is within the operating temperature range of the electronic component.

This application claims the benefit of U.S. Provisional Application No.:60/016,488 filing date Apr. 29, 1996.

BACKGROUND OF THE INVENTION

The present invention relates broadly to a heat transfer material whichis interposable between the thermal interfaces of a heat-generating,electronic component and a thermal dissipation member, such as a heatsink or circuit board, for the conductive cooling of the electroniccomponent. More particularly, the invention relates to aself-supporting, form-stable film which melts or softens at atemperature or range within the operating temperature range of theelectronic component to better conform to the thermal interfaces forimproved heat transfer from the electronic component to the thermaldissipation member.

Circuit designs for modern electronic devices such as televisions,radios, computers, medical instruments, business machines,communications equipment, and the like have become increasingly complex.For example, integrated circuits have been manufactured for these andother devices which contain the equivalent of hundreds of thousands oftransistors. Although the complexity of the designs has increased, thesize of the devices has continued to shrink with improvements in theability to manufacture smaller electronic components and to pack more ofthese components in an ever smaller area.

As electronic components have become smaller and more densely packed onintegrated boards and chips, designers and manufacturers now are facedwith the challenge of how to dissipate the heat which is ohmicly orotherwise generated by these components. Indeed, it is well known thatmany electronic components, and especially semiconductor components suchas transistors and microprocessors, are more prone to failure ormalfunction at high temperatures. Thus, the ability to dissipate heatoften is a limiting factor on the performance of the component.

Electronic components within integrated circuit traditionally have beencooled via forced or convective circulation of air within the housing ofthe device. In this regard, cooling fins have been provided as anintegral part of the component package or as separately attached theretofor increasing the surface area of the package exposed toconvectively-developed air currents. Electric fans additionally havebeen employed to increase the volume of air which is circulated withinthe housing. For high power circuits and the smaller but more denselypacked circuits typical of current electronic designs, however, simpleair circulation often has been found to be insufficient to adequatelycool the circuit components.

Heat dissipation beyond that which is attainable by simple aircirculation may be effected by the direct mounting of the electroniccomponent to a thermal dissipation member such as a “cold plate” orother heat sink. The heat sink may be a dedicated, thermally-conductivemetal plate, or simply the chassis of the device. However, and as isdescribed in U.S. Pat. No. 4,869,954, the faying thermal interfacesurfaces of the component and heat sink typically are irregular, eitheron a gross or a microscopic scale. When the interfaces surfaces aremated, pockets or void spaces are developed therebetween in which airmay become entrapped. These pockets reduce the overall surface areacontact within the interface which, in turn, reduces the efficiency ofthe heat transfer therethrough. Moreover, as it is well known that airis a relatively poor thermal conductor, the presence of air pocketswithin the interface reduces the rate of thermal transfer through theinterface.

To improve the efficiency of the heat transfer through the interface, alayer of a thermally-conductive material typically is interposed betweenthe heat sink and electronic component to fill in any surfaceirregularities and eliminate air pockets. Initially employed for thispurpose were materials such as silicone grease or wax filled with athermally-conductive filler such as aluminum oxide. Such materialsusually are semi-liquid or sold at normal room temperature, but mayliquefy or soften at elevated temperatures to flow and better conform tothe irregularities of the interface surfaces.

For example, U.S. Pat. No. 4,299,715 discloses a wax-like,heat-conducting material which is combined with another heat-conductingmaterial, such as a beryllium, zinc, or aluminum oxide powder, to form amixture for completing a thermally-conductive path from a heated elementto a heat sink. A preferred wax-like material is a mixture of ordinarypetroleum jelly and a natural or synthetic wax, such as beeswax, palmwax, or mineral wax, which mixture melts or becomes plastic at atemperature above normal room temperature. The material can beexcoriated or ablated by marking or rubbing, and adheres to the surfaceon which it was rubbed. In this regard, the material may be shaped intoa rod, bar, or other extensible form which may be carried in apencil-like dispenser for application.

U.S. Pat. No. 4,466,483 discloses a thermally-conductive,electrically-insulating gasket. The gasket includes a web or tape whichis formed of a material which can be impregnated or loaded with anelectronically-insulating, heat conducting material. The tape or webfunctions as a vehicle for holding the meltable material and heatconducting ingredient, if any, in a gasket-like form. For example, acentral layer of a solid plastic material may be provided, both sides ofwhich are coated with a meltable mixture of wax, zinc oxide, and a fireretardant.

U.S. Pat. No. 4,473,113 discloses a thermally-conductive,electrically-insulating sheet for application to the surface of anelectronic apparatus. The sheet is provided as having a coating on eachside thereof a material which changes state from a solid to a liquidwithin the operating temperature range of the electronic apparatus. Thematerial may be formulated as a meltable mixture of wax and zinc oxide.

U.S. Pat. No. 4,764,845 discloses a thermally-cooled electronic assemblywhich includes a housing containing electronic components. A heat sinkmaterial fills the housing in direct contact with the electroniccomponents for conducting heat therefrom. The heat sink materialcomprises a paste-like mixture of particulate microcrystalline materialsuch as diamond, boron nitride, or sapphire, and a filler material suchas a fluorocarbon or paraffin.

The greases and waxes of the aforementioned types heretofore known inthe art, however, generally are not self-supporting or otherwise formstable at room temperature and are considered to be messy to apply tothe interface surface of the heat sink or electronic component. Toprovide these materials in the form of a film which often is preferredfor ease of handling, a substrate, web, or other carrier must beprovided which introduces another interface layer in or between whichadditional air pockets may be formed. Moreover, use of such materialstypically involves hand application or lay-up by the electronicsassembler which increases manufacturing costs.

Alternatively, another approach is to substitute, a cured, sheet-likematerial for the silicone grease or wax material. Such materials may becompounded as containing one or more thermally-conductive particulatefillers dispersed within a polymeric binder, and may be provided in theform of cured sheets, tapes, pads, or films. Typical binder materialsinclude silicones, urethanes, thermoplastic rubbers, and otherelastomers, with typical fillers including aluminum oxide, magnesiumoxide, zinc oxide, boron nitride, and aluminum nitride.

Exemplary of the aforesaid interface materials is an alumina or boronnitride-filled silicone or urethane elastomer which is marketed underthe name CHO-THERM® by the Chomerics Division of Parker-Hannifin Corp.,Woburn, Mass. Additionally, U.S. Pat. No. 4,869,954 discloses a cured,form-stable, sheet-like, thermally-conductive material for transferringthermal energy. The material is formed of a urethane binder, a curingagent, and one or more thermally conductive fillers. The fillers mayinclude aluminum oxide, aluminum nitride, boron nitride, magnesiumoxide, or zinc oxide.

U.S. Pat. No. 4,782,893 discloses a thermally-conductive,electrically-insulative pad for placement between an electroniccomponent and its support frame. The pad is formed of a high dielectricstrength material in which is dispersed diamond powder. In this regard,the diamond powder and a liquid phase of the high dielectric strengthmaterial may be mixed and then formed into a film and cured. After thefilm is formed, a thin layer thereof is removed by chemical etching orthe like to expose the tips of the diamond particles. A thin boundarylayer of copper or other metal then is bonded to the top and bottomsurfaces of the film such that the exposed diamond tips extend into thesurfaces to provide pure diamond heat transfer paths across the film.The pad may be joined to the electronic component and the frame withsolder or an adhesive.

U.S. Pat. No. 4,965,699 discloses a printed circuit device whichincludes a memory chip mounted on a printed circuit card. The card isseparated from an associated cold plate by a layer of a siliconelastomer which is applied to the surface of the cold plate.

U.S. Pat. No. 4,974,119 discloses a heat sink assembly which includes anelectronic component supported on a printed circuit board in aspaced-apart relationship from a heat dispersive member. Athermally-conductive, elastomeric layer is interposed between the boardand the electronic component. The elastomeric member may be formed ofsilicone and preferably includes a filler such as aluminum oxide orboron nitride.

U.S. Pat. No. 4,979,074 discloses a printed circuit board device whichincludes a circuit board which is separated from a thermally-conductiveplate by a pre-molded sheet of silicone rubber. The sheet may be loadedwith a filler such as alumina or boron nitride.

U.S. Pat. No. 5,137,959 discloses a thermally-conductive, electricallyinsulating interface material comprising a thermoplastic or cross linkedelastomer filled with hexagonal boron nitride or alumina. The materialmay be formed as a mixture of the elastomer and filler, which mixturethen may be cast or molded into a sheet or other form.

U.S. Pat. No. 5,194,480 discloses another thermally-conductive,electricallyinsulating filled elastomer. A preferred filler is hexagonalboron nitride. The filled elastomer may be formed into blocks, sheets,or films using conventional methods.

U.S. Pat. Nos. 5,213,868 and 5,298,791 disclose a thermally-conductiveinterface material formed of a polymeric binder and one or morethermally-conductive fillers. The fillers may be particulate solids,such as aluminum oxide, aluminum nitride, boron nitride, magnesiumoxide, or zinc oxide. The material may be formed by casting or molding,and preferably is provided as a laminated acrylic pressure sensitiveadhesive (PSA) tape. At least one surface of the tape is provided ashaving channels or through-holes formed therein for the removal of airfrom between that surface and the surface of a substrate such as a heatsink or an electronic component.

U.S. Pat. No. 5,321,582 discloses an electronic component heat sinkassembly which includes a thermally-conductive laminate formed ofpolyamide which underlies a layer of a boron nitride-filled silicone.The laminate is interposed between the electronic component and thehousing of the assembly.

Sheet-like materials of the above-described types have garnered generalacceptance for use as interface materials in conductively-cooledelectronic component assemblies. For some applications, however, heavyfastening elements such as springs, clamps, and the like are required toapply enough force to conform these materials to the interface surfacesto attain enough surface for efficient thermal transfer. Indeed, forcertain applications, materials such as greases and waxes which liquefy,melt, or soften at elevated temperature sometimes as preferred as betterconforming to the interface surfaces. It therefore will be appreciatedthat further improvements in these types of interface materials andmethods of applying the same would be well-received by the electronicsindustry. Especially desired would be a thermal interface material whichis self-supporting and form-stable at room temperature, but which issoftenable or meltable at temperatures within the operating temperaturerange of the electronic component to better conform to the interfacesurfaces.

BROAD STATEMENT OF THE INVENTION

The present invention is directed to a heat transfer material which isinterposable between the thermal interfaces of a heat-generating,electronic component and a thermal dissipation member. The material isof the type which melts or softens at a temperature or range within theoperating temperature range of the electronic component to betterconform to the thermal interfaces for improved heat transfer from theelectronic component to the thermal dissipation member. Unlike thegreases or waxes of such type heretofore known in the art, however, theinterface material of the present invention is form-stable andself-supporting at room temperature. Accordingly, the material may beformed into a film or tape which may be applied using automatedequipment to, for example, the interface surface of a thermaldissipation member such as a heat sink. In being self-supporting, no webor substrate need be provided which would introduce another layer intothe interface between which additional air pockets could be formed.

It therefore is a feature of the present invention to provide for theconductive cooling a heat-generating electronic component. The componenthas an operating temperature range above normal room temperature and afirst heat transfer surface disposable in thermal adjacency with asecond heat transfer surface of an associated thermal dissipation memberto define an interface therebetween. A thermally-conductive material isprovided which is form-stable at normal room temperature in a firstphase and conformable in a second phase to substantially fill theinterface. The material, which has a transition temperature from thefirst phase to the second phase within the operating temperature rangeof the electronic component, is formed into a self-supporting layer. Thelayer is applied to one of the heat transfer surfaces, which surfacesthen are disposed in thermal adjacency to define the interface. Theenergization of the electronic component is effective to heat the layerto a temperature which is above the phase transition temperature.

It is a further feature of the invention to provide athermally-conductive interface for conductively cooling aheat-generating electronic component having an associated thermaldissipation member such as a heat sink. The interface is formed as aself-supporting mono-layer of a thermally-conductive material which isform-stable at normal room temperature in a first phase andsubstantially conformable in a second phase to the interface surfaces ofthe electronic component and thermal dissipation member. The materialhas a transition temperature from the first phase to the second phasewhich is within the operating temperature range of the electroniccomponent.

Advantages of the present invention include a thermal interface materialwhich melts of softens to better conform to the interfaces surfaces, butwhich is self-supporting and form-stable at room temperature for ease ofhandling and application. Further advantages include an interfacematerial which may be formed into a film or tape without a web or othersupporting substrate, and which may be applied using automated methodsto, for example, the interface surface of a thermal dissipation member.Such member then may be shipped to a manufacturer for directinstallation into a circuit board to thereby obviate the need for handlay-up of the interface material. Still further advantages include athermal interface formulation which may be tailored to providecontrolled thermal and viscometric properties. These and otheradvantages will be readily apparent to those skilled in the art basedupon the disclosure contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a fragmentary, cross-sectional view of an electrical assemblywherein a heatgenerating electronic component thereof is conductivelycooled in accordance with the present invention via the provision of aninterlayer of a thermally-conductive material within the thermalinterace between the heat transfer surfaces of the component and anassociated thermal dissipation member;

FIG. 2 is a view of a portion of the thermal interface of FIG. 1 whichis enlarged to detail the morphology thereof,

FIG. 3 is a cross-sectional end view which shows thethermally-conductive material of FIG. 1 as coated as a film layer onto asurface of a release sheet, which sheet is rolled to facilitate thedispensing of the film; and

FIG. 4 is a view of a portion of the film and release sheet roll of FIG.3 which is enlarged to detail the structure thereof

The drawings will be described further in connection with the followingDetailed Description of the Invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein corresponding reference charactersindicate corresponding elements throughout the figures, shown generallyat 10 in FIG. 1 is an electrical assembly which includes aheat-generating digital or analog electronic component 12, supported onan associated printed circuit board (PCB) or other substrate, 14.Electrical component 12 may be an integrated microchip, microprocessor,transistor, or other semiconductor, or an ohmic or other heat-generatingsubassembly such as a diode, relay, resistor, transformer, amplifierdiac, or capacitor. Typically, component 12 will have an operatingtemperature range of from about 60-80° C. For the electrical connectionof component 12 to board 14, a pair of leads or pins, 16a and 16b, areprovided as extending from either end of component 12 into a soldered orother connection with board 14. Leads 16 additionally may supportcomponent 12 above board 14 to define a gap, represented at 17, of about3 mils (75 microns) therebetween. Alternatively, component 12 may bereceived directly on board 14.

As supported on board 14, electronic component 12 presents a first heattransfer surface, 18, which is disposable in a thermal, spaced-apartadjacency with a corresponding second heat transfer surface, 22, of anassociated thermal dissipation member, 20. Dissipation member 20 isconstructed of a metal material or the like having a heat capacityrelative to that of component 12 to be effective is dissipating thermalenergy conducted or otherwise transferred therefrom. For purposes of thepresent illustration, thermal dissipation member 20 is shown as a heatsink having a generally planar base portion, 24, from which extends aplurality of cooling fins, one of which is referenced at 26. Withassembly 10 configured as shown, fins 26 assist in the convectivecooling of component 12, but alternatively may be received within anassociated cold plate or the like, not shown, for further conductivedissipation of the thermal energy transferred from component 12.

The disposition of first heat transfer surface 18 of electroniccomponent 12 in thermal adjacency with second heat transfer surface 22of dissipation member 20 defines a thermal interface, represented at 28,therebetween. A thermally-conductive interlayer, 30, is interposedwithin interface 28 between heat transfer surfaces 18 and 22 forproviding a conductive path therethrough for the transfer of thermalenergy from component 12 to dissipation member 20. Such path may beemployed without or in conjunction with convective air circulation foreffecting the cooling of component 12 and ensuring that the operatingtemperature thereof is maintained below specified limits.

Although thermal dissipation member 20 is shown to be a separate heatsink member, board 14 itself may be used for such purpose byalternatively interposing interlayer 30 between surface 32 thereof andcorresponding surface 34 of electronic component 12. In eitherarrangement, a clip, spring, or clamp or the like (not shown)additionally may be provided for applying an external force, representedat 32, of from about 1-2 lbs_(f) for improving the interface areacontact between interlayer 30 and surfaces 18 and 22 or 32 and 34.

In accordance with the precepts of the present invention, interlayer 30is formed of a self-supporting film, sheet, or other layer of athermally-conductive material. By “self-supporting,” it is meant thatinterlayer 30 is free-standing without the support of a web or substratewhich would introduce another layer into the thermal interface betweenair pockets could be formed. Typically, the film or sheet of interlayer30 will have a thickness of from about 1-10 mils (25-250 microns)depending upon the particular geometry of assembly 10.

The thermally-conductive material forming interlayer 30 is formulated tobe form-stable at normal room temperature, i.e., about 25° C., in afirst phase, which is solid, semi-solid, glassy, or crystalline, but tobe substantially conformable in a second phase, which is a liquid,semi-liquid, or otherwise viscous melt, to interface surfaces 18 and 22of, respectively, electronic component 12 and thermal dissipation member20. The transition temperature of the material, which may be its meltingor glass transition temperature, is preferably from about 60 or 70° C.to about 80° C., and is tailored to fall within the operatingtemperature of electronic component 12.

Further in this regard, reference may be had to FIG. 2 wherein anenlarged view of a portion of interface 28 is illustrated to detail theinternal morphology thereof during the enerigization of electroniccomponent 12 effective to heat interlayer 30 to a temperature which isabove its phase transition temperature. Interlayer 30 accordingly isshown to have been melted or otherwise softened from a form-stable solidor semi-solid phase into a flowable or otherwise conformable liquid orsemi-liquid viscous phase which may exhibit relative intermolecularchain movement. Such viscous phase provides increased surface areacontact with interface surfaces 18 and 22, and substantially completelyfills interface 28 via the exclusion of air pockets or other voidstherefrom to thereby improve both the efficiency and the rate of heattransfer through interface. Moreover, as depending on, for example, themelt flow index or viscosity of interlayer 30 and the magnitude of anyapplied external pressure 36 (FIG. 1), the interface gap betweensurfaces 18 and 22 may be narrowed to further improve the efficiency ofthe thermal transfer therebetween. Any latent heat associated with thephase change of the material forming interlayer 30 additionallycontributes to the cooling of component 12.

Unlike the greases or waxes of such type heretofore known in the art,however, interlayer of the present invention advantageously isform-stable and self-supporting at room temperature. Accordingly, and asis shown generally at 40 in FIG. 3, interlayer 30 advantageously may beprovided in a rolled, tape form to facilitate its application to thesubstrate by an automated process. As may be better appreciated withadditional reference to FIG. 4 wherein a portion, 42, of tape 40 isshown in enhanced detail, tape 40 may be formed by applying a film ofinterlayer 30 to a length of face stock, liner, or other release sheet,44. Interlayer 30 may be applied to a surface, 46, of release sheet 44in a conventional manner, for example, by a direct process such asspraying, knife coating, roller coating, casting, drum coating, dipping,or like, or an indirect transfer process utilizing a silicon releasesheet. A solvent, diluent, or other vehicle may be provided to lower theviscosity of the material forming interlayer 30. After the material hasbeen applied, the release sheet may be dried to flash the solvent andleave an adherent, tack-free film, coating, or other residue of thematerial thereon.

As is common in the adhesive art, release sheet 44 may be provided as astrip of a waxed, siliconized, or other coated paper or plastic sheet orthe like having a relatively low surface energy so as to be removablewithout appreciable lifting of interlayer 30 from the substrate to whichit is ultimately applied. Representative release sheets include facestocks or other films of plasticized polyvinyl chloride, polyesters,cellulosics, metal foils, composites, and the like.

In the preferred embodiment illustrated, tape 40 may be sectioned tolength, and the exposed surface, 48, of interlayer 30 may be applied tointerface surface 22 of dissipation member 20 (FIG. 1) prior to itsinstallation in assembly 10. In this regard, interlayer exposed surface48 may be provided as coated with a thin film of a pressure sensitiveadhesive or the like for adhering interlayer 30 to dissipation member20. Alternatively, interface surface 22 of dissipation member 20 may beheated to melt a boundary layer of interlayer surface 48 for itsattachment via a “hot-melt” mechanism.

With tape 40 so applied and with release sheet 44 protecting theunexposed surface, 50, of interlayer 30, dissipation member 20 (FIG. 1)may be packaged and shipped as an integrated unit to an electronicsmanufacturer, assembler, or other user. The user then simply may removerelease sheet 44 to expose surface 50 of interlayer 30, position surface50 on heat transfer surface 18 of electronic component 12, and lastlyapply a clip or other another means of external pressure to disposeinterlayer surface 50 in an abutting, heat transfer contact or otherthermal adjacency with electronic component surface 18.

In one preferred embodiment, interlayer 30 is formulated as aform-stable blend of: (a) from about 25 to 50% by weight of a pressuresensitive adhesive (PSA) component having a melting temperature of fromabout 90-100° C.; (b) from about 50 to 75% by weight of an α-olefinic,thermoplastic component having a melting temperature of from about50-60° C.; and (c) from about 20 to 80% by weight of one or morethermally-conductive fillers. “Melting temperature” is used herein inits broadest sense to include a temperature or temperature rangeevidencing a transition from a form-stable solid, semi-solid,crystalline, or glassy phase to a flowable liquid, semi-liquid, orotherwise viscous phase or melt which may be characterized as exhibitingintermolecular chain rotation.

The PSA component generally may be of an acrylic-based, hot-melt varietysuch as a homopolymer, copolymer, terpolymer, interpenetrating network,or blend of an acrylic or (meth)acrylic acid, an acrylate such as butylacrylate, and/or an amide such as acrylamide. The term “PSA” is usedherein in its conventional sense to mean that the component isformulated has having a glass transition temperature, surface energy,and other properties such that it exhibits some degree of tack at normalroom temperature. Acrylic hot-melt PSAs of such type are marketedcommercially by Heartland Adhesives, Germantown, Wis., under the tradedesignations “H600” and “H251.”

The α-olefinic thermoplastic component preferably is a polyolefin whichmay be characterized as a “low melt” composition. A representativematerial of the preferred type is an amorphous polymer of a C₁₀ orhigher alkene which is marketed commercially by Petrolite Corporation,Tulsa, Okla., under the trade designation “Vybar® 260.” Such materialmay be further characterized as is set forth in Table 1.

TABLE 1 Physical Properties of Representative Olefinic Polymer Component(Vybar ® 260) Molecular Weight 2600 g/mol Melting Point (ASTM D 36) 130°F. (54° C.) Viscosity (ASTM D 3236) 357.5 cP @ 210° F. (99° C.)Penetration (ASTM D 1321) 12 mm @ 77° F. (25° C.) Density (ASTM D 1168)@ 75° F. (24° C.) 0.90 g/cm³ @ 200° F. (93° C.) 0.79 g/cm³ Iodine Number(ASTM D 1959) 15

By varying the ratio within the specified limits of the PSA to thethermoplastic component, the thermal and viscometric properties of theinterlayer formulation may be tailored to provide controlled thermal andviscometric properties. In particular, the phase transition temperatureand melt flow index or viscosity of the formulation may be selected foroptimum thermal performance with respect to such variables as theoperating temperature of the heat generating electronic component, themagnitude of any applied external pressure, and the configuration of theinterface.

In an alternative embodiment, a paraffinic wax or other natural orsynthetic ester of a long-chain (C₁₆ or greater) carboxylic acid andalcohol having a melting temperature of from about 60-70° C. may besubstituted for the thermoplastic and PSA components to comprise about20-80% by weight of the formulation. A preferred wax is marketedcommercially by Bareco Products of Rock Hill, S.C. under the tradedesignation “Ultraflex® Amber,” and is compounded as a blend ofclay-treated microcrystalline and amorphous constituents. Such wax isadditionally characterized in Table 2 which follows.

TABLE 2 Physical Properties of Representative Paraffinic Wax Component(Ultraflex ® Amber) Melting Point (ASTM D 127) 156° F. (69° C.)Viscosity (ASTM D 3236) 13 cP @ 210° F. (99° C.) Penetration (ASTM D1321) @ 77° F. (25° C.) 29 mm @ 110° F. (43° C.) 190 mm Density (ASTM D1168) @ 75° F. (25° C.) 0.92 g/cm³ @ 210° F. (99° C.) 0.79 g/cm³

In either of the described embodiments, the resin or wax components forma binder into which the thermally-conductive filler is dispersed. Thefiller is included within the binder in a proportion sufficient toprovide the thermal conductivity desired for the intended application.The size and shape of the filler is not critical for the purposes of thepresent invention. In this regard, the filler may be of any generalshape including spherical, flake, platelet, irregular, or fibrous, suchas chopped or milled fibers, but preferably will be a powder or otherparticulate to assure uniform dispersal and homogeneous mechanical andthermal properties. The particle size or distribution of the fillertypically will range from between about 0.25-250 microns (0.01-10 mils),but may further vary depending upon the thickness of interface 28 and/orinterlayer 30.

It additionally is preferred that the filler is selected as beingelectrically-nonconductive such that interlayer 30 may provide anelectrically-insulating but thermally-conductive barrier betweenelectronic component 12 and thermal dissipation member 20. Suitablethermally-conductive, electrically insulating fillers include boronnitride, alumina, aluminum oxide, aluminum nitride, magnesium oxide,zinc oxide, silicon carbide, beryllium oxide, and mixtures thereof Suchfillers characteristically exhibit a thermal conductivity of about 25-50W/m-°K.

Additional fillers and additives may be included in interlayer 30 to theextent that the thermal conductivity and other physical propertiesthereof are not overly compromised. As aforementioned, a solvent orother diluent may be employed during compounding to lower the viscosityof the material for improved mixing and delivery. Conventional wettingopacifying, or anti-foaming agents, pigments, flame retardants, andantioxidants also may be added to the formulation depending upon therequirements of the particular application envisioned. The formulationmay be compounded in a conventional mixing apparatus.

Although not required, a carrier or reinforcement member (not shown)optionally may be incorporated within interlayer 30 as a separateinternal layer. Conventionally, such member may be provided as a filmformed of a thermoplastic material such as a polyimide, or as a layer ofa woven fiberglass fabric or an expanded aluminum mesh. Thereinforcement further supports the interlayer to facilitate its handlingat higher ambient temperatures and its die cutting into a variety ofgeometries.

The Example to follow, wherein all percentages and proportions are byweight unless otherwise expressly indicated, is illustrative of thepracticing of the invention herein involved, but should not be construedin any limiting sense.

EXAMPLE

Master batches representative of the interlayer formulations of thepresent invention were compounded for characterization according to thefollowing schedule:

TABLE 3 Representative Interlayer Formulations Ultraflex ® SampleVybar ® 260¹ H600² Amber³ Filler (wt. %) No. (wt. %) (wt. %) (wt. %) Bn⁴ZnO₂ ⁵ Al⁶ 3-1 45 22 33 3-2 47 17 36 3-3 47 17 6 30 3-6 40 60 3-7 40 1941 3-8 50 25 25  3-10 34 16 50 5-1 67 33 ¹α-olefinic thermoplastic,Petrolite Corp., Tulsa, OK ²acrylic PSA, Heartland Adhesives,Germantown, WI ³paraffinic wax, Bareco Products Corp. Rock Hill, SC⁴Boron nitride, HCP particle grade, Advanced Ceramics, Cleveland, OH⁵Zinc oxide, Midwest Zinc, Chicago, IL; Wittaker, Clark & Daniels, Inc.,S. Plainfield, NJ ⁶Alumina, R1298, Alcan Aluminum, Union, NJ

The Samples were thinned to about 30-70% total solids with toluene orxylene, cast, and then dried to a film thickness of from about 2.5 to 6mils. When heated to a temperature of between about 55-65° C., theSamples were observed to exhibit a conformable grease or paste-likeconsistency. The following thermal properties were measured and comparedwith conventional silicone grease (Dow 340, Dow Corning, Midland, Mich.)and metal foil-supported wax (Crayotherm™, Crayotherm Corp., Anaheim,Calif.) formulations:

TABLE 4 Thermal Properties of Representative and Comparative InterlayerFormulations Thermal Thermal Sample Formu- Filler Thickness Impedance⁵Conductivity⁵ No. lation (wt. %) (mills) (° C.-in/w) (w/m-° K.) 3-1blend 62% Al 6 0.14 1.7 3-2 blend 62% Al 4 0.12 1.3 3-3 blend 62% 4 0.091.7 Al/BN 3-6 wax² 60% Al 2.5 0.04 2.3 5-1 wax 50% 4 0.10 1.5 BN 3-7blend 62% 4 0.14 1.1 ZnO₂ 3-8 blend 30% 2.5 0.07 1.5 BN 3-10 blend 70% 30.12 0.95 ZnO₂ Crayo- wax/foil³ ZnO₂ 2.5 0.11 0.93 therm 3-2 blend 62%Al 5 0.26 0.74 3-6 wax 60% Al 5 0.30 0.65 5-1 wax 50% 5 0.12 1.64 BN Dowgrease⁴ ZnO₂ 5 (true⁶) 0.36 0.54 340 ¹blend of Vybar ® and H600²Ultraflex ® Amber ³metal foil-supported wax ⁴silicone grease ⁵measuredusing from about 10-300 psi applied external pressure ⁶spacers used tocontrol thickness

The foregoing results confirm that the interlayer formulations of thepresent invention retain the preferred conformal and thermal propertiesof the greases and waxes heretofore known in the art. However, suchformulations additionally are form-stable and self-supporting at roomtemperature, thus affording easier handling and application andobviating the necessity for a supporting substrate, web, or othercarrier.

As it is anticipated that certain changes may be made in the presentinvention without departing from the precepts herein involved, it isintended that all matter contained in the foregoing description shall beinterpreted as illustrative and not in a limiting sense. All referencescited herein are expressly incorporated by reference.

1. A method of conductively cooling a heat-generating electroniccomponent having an operating temperature range above normal roomtemperature and a first heat transfer surface disposable in thermaladjacency with a second heat transfer surface of a thermal dissipationmember to define an interface therebetween, said method comprising thesteps of: (a) providing a thermally-conductive material which isform-stable at normal room temperature in a first phase and conformablein a second phase to substantially fill said interface, said materialhaving a transition temperature from said first phase to said secondphase within the operating temperature range of said electroniccomponent, and said material consisting essentially of at least oneresin or wax component blended with at least one thermally-conductivefiller; (b) forming said material into a self-supporting andfree-standing film layer, said layer consisting essentially of saidmaterial and having a thickness of from about 1-10 mils; (c) applyingsaid layer to one of said heat transfer surfaces; (d) disposing saidheat transfer surfaces in thermal adjacency to define said interface;and (e) energizing said electronic component effective to heat saidlayer to a temperature which is above said phase transition temperature.2. The method of claim 1 further comprising an additional step betweensteps (d) and (e) of applying an external force to at least one of saidheat transfers defining said interface.
 3. The method of claim 1 whereinsaid thermal dissipation member is a heat sink or a circuit board. 4.The method of claim 1 wherein said layer is applied in step (c) to theheat transfer surface of said electronic component.
 5. The method ofclaim 1 wherein said self-supporting layer is formed in step (b) bycoating a film of said material onto a surface of a release sheet, andwherein said layer is applied in step (c) by adhering said film to oneof said heat transfer and removing said release sheet to expose saidfilm.
 6. The method of claim 1 wherein said material is provided in step(a) as consisting essentially of a blend of: (i) from about 20 to 80% byweight of a paraffinic wax component having a melting temperature offrom about 60-70° C.; and (ii) from about 20 to 80% by weight of one ormore thermally-conductive fillers.
 7. The method of claim 6 wherein saidmaterial has a phase transition temperature of from about 60-80° C. 8.The method of claim 6 wherein said one or more thermally-conductivefillers is selected from the group consisting of boron nitride, alumina,aluminum oxide, aluminum nitride, magnesium oxide, zinc oxide, siliconcarbide, beryllium oxide, and mixtures thereof.
 9. Athermally-conductive interface for interposition between aheat-generating electronic component having an operating temperaturerange above normal room temperature and a first heat transfer surfacedisposable in thermal adjacency with a second heat transfer surface of athermal dissipation member, said interface comprising a self-supportingand free-standing film layer having a thickness of from about 1-10 milsand consisting essentially of a thermally-conductive material which isform-stable at normal room temperature in a first phase andsubstantially conformable in a second phase to said interface surfaces,said material having a transition temperature from said first phase tosaid second phase within the operating temperature range of saidelectronic component, and said material consisting essentially of atleast one resin or wax component blended with at least onethermally-conductive filler.
 10. The interface of claim 9 which iscoated as a film onto a surface of a release sheet.
 11. The interface ofclaim 9 wherein said material consisting essentially of a blend of: (a)from about 20 to 80% by weight of a paraffinic wax component having amelting temperature of from about 60-70° C.; and (b) from about 20 to80% by weight of one or more thermally-conductive fillers.
 12. Theinterface of claim 11 wherein said material has a phase transitiontemperature of from about 60-80° C.
 13. The interface of claim 11wherein said one or more thermally-conductive fillers is selected fromthe group consisting of boron nitride, alumina, aluminum oxide, aluminumnitride, magnesium oxide, zinc oxide, silicon carbide, beryllium oxide,and mixtures thereof.
 14. A method of conductively cooling aheat-generating electronic component having an operating temperaturerange above normal room temperature and a first heat transfer surfacedisposable in thermal adjacency with a second heat transfer surface of athermal dissipation member to define an interface therebetween, saidmethod comprising the steps of: (a) providing a thermally-conductivematerial which is form-stable at normal room temperature in a firstphase and conformable in a second phase to substantially fill saidinterface, said material having a transition temperature from said firstphase to said second phase within the operating temperature range ofsaid electronic component and comprising a blend of: (i) from about 25to 50% by weight of an acrylic pressure sensitive adhesive componenthaving a melting temperature of from about 90-100° C.; (ii) from about50 to 75% by weight of an α-olefinic, thermoplastic component having amelting temperature of from about 50-60° C.; and (iii) from about 20 to80% by weight of one or more thermally-conductive fillers; (b) formingsaid material into a self-supporting layer; (c) applying said layer toone of said heat transfer surfaces; (d) disposing said heat transfersurfaces in thermal adjacency to define said interface; and (e)energizing said electronic component effective to heat said layer to atemperature which is above said phase transition temperature.
 15. Themethod of claim 14 wherein said material has a phase transitiontemperature of from about 70-80° C.
 16. The method of claim 14 whereinsaid one or more thermally-conductive fillers is selected from the groupconsisting of boron nitride, alumina, aluminum oxide, aluminum nitride,magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, andmixtures thereof.
 17. A thermally-conductive interface for interpositionbetween a heat-generating electronic component having an operatingtemperature range above normal room temperature and a first heattransfer surface disposal in thermal adjacency with a second heattransfer surface of a thermal dissipation member, said interfacecomprising a self-supporting layer of a thermally-conductive materialwhich is form-stable at normal room temperature in a first phase andsubstantially conformable in a second phase to said interface surfaces,said material having a transition temperature from said first phase tosaid second phase within the operating temperature range of saidelectronic component, and comprising a blend of: (a) from about 25 to50% by weight of an acrylic pressure sensitive adhesive component havinga melting temperature of from about 90-100° C.; (b) from about 50 to 75%by weight of an α-olefinic, thermoplastic component having a meltingtemperature of from about 50-60° C.; and (c) from about 20 to 80% byweight of one or more thermally-conductive fillers.
 18. The interface ofclaim 17 wherein said material has a phase transition temperature offrom about 70-80° C.
 19. The interface of claim 17 wherein said one ormore thermally-conductive fillers is selected from the group consistingof boron nitride, alumina, aluminum oxide, aluminum nitride, magnesiumoxide, zinc oxide, silicon carbide, beryllium oxide, and mixturesthereof.